Microprocessor circuits, systems, and methods implementing a loop and/or stride predicting load target buffer

• US 5,953,512 A
• Filed: 12/29/1997
• Issued: 09/14/1999
• Est. Priority Date: 12/31/1996
• Status: Expired due to Term

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Shiell, Jonathan H., Cai, George Z. N.
• Primary Examiner(s): VU, VIET DUY
• Application Number:
  US08/999,087
• Time in Patent Office: 624 Days
• Field of Search: 395/381, 395/383, 395/390, 395/586, 395/587, 395/583, 395/584, 395/585, 395/800.23, 711/137, 711/204, 711/205, 711/217
• US Class Current: 712/205
• CPC Class Codes: G06F 9/3455 using stride
• CPC Class Codes: G06F 9/3806 using address prediction, e.g. return stack, branch history buffer
• CPC Class Codes: G06F 9/383 Operand prefetching cache prefetching G06F12/0862
• CPC Class Codes: G06F 9/3848 using hybrid branch prediction, e.g. selection between prediction techniques

Abstract

A load target circuit (56) with a plurality of entries (56₁). Each the plurality of entries in the load target circuit comprises a value (ADDRESS TAG) for corresponding the line to a data fetching instruction. Additionally, each load target circuit line also includes a plurality of pointers (POINTER A, POINTER B, POINTER C). Each of the plurality of pointers is for storing a target data address corresponding to an incident of the data fetching instruction.

Claims

1. A method of operating a microprocessor having an instruction pipeline, comprising the steps of:

receiving a data fetching instruction in the instruction pipeline over a plurality of incidents, wherein each of the plurality of incidents produces a corresponding plurality of actual target data addresses for the data fetching instruction;

forming an entry in a load target buffer on the microprocessor in response to a first one of the plurality of incidents, wherein the entry corresponds to the data fetching instruction;

in response to the plurality of incidents, encoding in the entry a prediction of a target data address, wherein the prediction is based on the corresponding plurality of actual target data addresses for the data fetching instruction;

wherein the prediction is selected from a first prediction type and a second prediction type;

wherein the first prediction type is a loop mode such that the plurality of actual target data addresses form a loop sequence having a beginning loop target data address and passing to an ending loop target data address and repeating by returning to the beginning loop target data address; and

wherein the second prediction type is a stride mode such that the plurality of actual target data addresses pass from a beginning stride target data address and pass to a number of additional successive addresses, wherein the target data address of each of the successive target addresses has a common distance between itself and a target data address of an immediately preceding target data address.

2. A load target circuit, comprising:

a plurality of entries, wherein each of the plurality of entries comprises;

a value for corresponding the line to a data fetching instruction; and

a plurality of pointers, wherein each of the plurality of pointers is for storing a target data address corresponding to an incident of the data fetching instruction, wherein each of the plurality of entries further comprises a next pointer value for identifying which one of the plurality of pointers provides the target data address for a given incident of the data fetching instruction.

3. The load target circuit of claim 2, wherein each of the plurality of entries further comprises a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers.

4. The load target circuit of claim 2:

wherein each of the plurality of entries further comprises;

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

wherein each of the plurality of pointers is logically arranged in a sequential and circular order; and

wherein, for a given one of the plurality of entries, in response to a given one of the plurality of control values indicating a loop mode, the next pointer is incremented from a first one of the plurality of pointers corresponding to the given one of the plurality of control values to a second one of the plurality of pointers in the sequential and circular order after issuing a prefetch request corresponding to the target data address of the first one of the plurality of pointers.

5. The load target circuit of claim 2:

wherein each of the plurality of entries further comprises;

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

wherein, for a given one of the plurality of entries, in response to a given one of the plurality of control values indicating a loop mode, the next pointer is adjusted from a first one of the plurality of pointers corresponding to the given one of the plurality of control values to a second one of the plurality of pointers after issuing a prefetch request corresponding to the target data address of the first one of the plurality of pointers.

6. The load target circuit of claim 2 wherein each of the plurality of entries further comprises:

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

a stride threshold value for indicating a number of data target addresses in a stride sequence.

7. The load target circuit of claim 6:

further comprising circuitry for issuing prefetch requests; and

wherein each entry of the plurality of entries further comprises a stride counter for counting the number of data target addresses issued in a sequence of prefetch requests corresponding to the entry.

8. The load target circuit of claim 6:

further comprising circuitry for issuing prefetch requests; and

wherein, for a given one of the plurality of entries, in response to successive incidents of the data fetching instruction, and further in response to one of the plurality of control values indicating a stride mode, the circuitry for issuing prefetch requests issues a number of prefetch requests equal to the number of data target addresses in the stride sequence indicated by the stride threshold value.

9. The load target circuit of claim 2 wherein each of the plurality of entries further comprises:

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

a stride length value for indicating a stride length between successive data target addresses in a stride sequence.

10. The load target circuit of claim 9:

further comprising circuitry for issuing prefetch requests;

wherein, for a given one of the plurality of entries, in response to successive incidents of the data fetching instruction, and further in response to one of the plurality of control values indicating a stride mode, the circuitry for issuing prefetch requests issues successive prefetch requests;

wherein each of the successive prefetch requests includes a data target address; and

wherein the data target address of each of the successive prefetch requests after a first of the successive prefetch requests has the stride length as the difference between itself and a data target address of an immediately preceding prefetch request.

11. The load target circuit of claim 2 wherein each of the plurality of entries further comprises:

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

a stride threshold value for indicating a number of data target addresses in a stride sequence; and

a stride length value for indicating a stride length between successive data target addresses in a stride sequence.

12. The load target circuit of claim 2:

further comprising circuitry for issuing prefetch requests; and

wherein each of the plurality of entries further comprises;

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

a stride threshold value for indicating a number of data target addresses in a stride sequence; and

wherein, for a given one of the plurality of entries, in response to successive incidents of the data fetching instruction, and further in response to one of the plurality of control values indicating a stride mode, the circuitry for issuing prefetch requests issues a number of prefetch requests equal to the number of data target addresses in the stride sequence indicated by the stride threshold value; and

wherein, for a given one of the plurality of entries, in response to a given one of the plurality of control values indicating a loop mode, the next pointer is incremented from a first one of the plurality of pointers corresponding to the given one of the plurality of control values to a second one of the plurality of pointers after issuing a prefetch request corresponding to the target data address of the first one of the plurality of pointers.

13. The load target circuit of claim 2:

wherein each of the plurality of entries further comprises

a plurality of control values, wherein each one of the plurality of control values corresponds to one of the plurality of pointers; and

circuitry for maintaining the next pointer value to a constant value to identify one of the plurality of pointers for multiple incidents of the data fetching instruction thereby creating a same address loop mode.

14. The load target circuit of claim 13:

further comprising circuitry for issuing prefetch requests;

wherein, for a given one of the plurality of entries, in response to successive incidents of the data fetching instruction, and further in response to the same address loop mode, the circuitry for issuing prefetch requests issues successive prefetch requests having the same target data address.

15. The load target circuit of claim 2, wherein the plurality of pointers is three pointers.

16. The load target circuit of claim 2, wherein the data fetching instruction is a load instruction.

17. The load target circuit of claim 2, wherein the data fetching instruction is a store instruction.